No-backlash gearing mechanism

ABSTRACT

A no back-lash gearing mechanism comprises meshing toothed elements. At least one of the toothed elements has a number of teeth having an undulated, &#34;S&#34;-shaped, transverse section. The teeth are arranged such that, when the toothed elements mesh, one of the teeth having the undulated form makes at least one point of contact with each of two opposing adjacent teeth of the other element thus preventing back-lash.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a no-backlash gearing mechanism.

2. Discussion of the Related Art

A no-backlash gearing mechanism is disclosed in United Kingdom patentspecification No. 1 251 941 in which the teeth of at least one of a pairof meshing toothed elements are formed of elastic rubber or plasticsmaterial and may additionally be formed with slots or bores to improvetheir deformability. By urging the toothed elements together whenmeshing, the deformable teeth of one element fully mesh with the teethof the other element without back-lash. However, this gearingarrangement is only suitable for very light loads since, due to thedeformability of the teeth, slippage of the meshing toothed elements canoccur.

European patent specification No. 0 084 197 also offers a solution tothe problem of back-lash in gearing mechanisms. The gearing mechanism ofEP 0 084 197 comprises a pair of meshing toothed elements, each of whichhave teeth of transverse asymmetrical shape which, when meshing, areurged into contact with each other in a direction normal to the plane ofrotation of the toothed elements. In addition, the teeth of the toothedelements may be slotted to improve their deformability and thus ensureno-backlash. This arrangement necessitates the inclusion of means forurging the meshing toothed elements into contact in a direction normalto the plane of rotation of the toothed elements.

United Kingdom specification No. 1 202 806 also offers a solution to theproblem of back-lash in gearing mechanisms. The solution discussed iseven more similar to that of GB 1 251 941 than that of EP 0 084 197. Thegearing mechanism of GB 1 202 806, whilst being an internal/ring gearmechanism, utilises the principle of having a first meshing toothedelement with teeth formed of deformable material urged into meshingengagement with a second toothed element, wherein the deformable teeth,when meshing, deform to completely mesh with the teeth of the secondtoothed element thereby preventing back-lash. This gearing mechanism isalso susceptible to slippage due to the high deformability of the teeth.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to obviate and mitigate theaforesaid disadvantages of the prior art gearing mechanisms by providinga gearing mechanism for eliminating back-lash without undulycompromising the drive capabilities of the mechanism so formed.

According to the present invention there is provided a gearing mechanismcomprising meshing toothed elements wherein at least some of the teethof one of said elements have an undulated longitudinal transversesection whereby the elements can be meshed such that at any one time oneof the teeth having an undulated longitudinal transverse section makesat least one point of contact with each of two opposing adjacent teethof the other element.

Preferably, all of the teeth of one of the elements have an undulatedlongitudinal transverse section.

Preferably further, all of the teeth of the other element have acorresponding conversely undulated longitudinal transverse section.

Preferably also, the teeth of the meshing toothed elements having anundulated longitudinal transverse section have a regularly undulatedform.

The teeth of the meshing toothed elements may be resiliently deformable.

The meshing toothed elements may each be generally in the form of gearwheels.

Alternatively, one of said meshing toothed elements may be a rack, thusproviding a gearing mechanism in the form of a rack and pinion.

Preferably, the pitch of teeth on one meshing toothed element exceeds bya small amount the spaces between teeth of the other meshing element.

The gearing mechanism may comprise a spur gear, bevel gear or internalgear mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features of the present invention will be morereadily understood from the following description of preferredembodiments, by way of example thereof, with reference to theaccompanying drawings, of which:

FIG. 1a is a diagrammatic view of the periphery of a pair of involutegears of known construction, partially developed in a plane andappropriately labeled "PRIOR ART";

FIG. 1b is a profile of the teeth of each of the gears of FIG. 1a andappropriately labeled "PRIOR ART";

FIG. 2 is a perspective view of a gearing mechanism in accordance with afirst embodiment of the present invention;

FIG. 3a is a diagrammatic view of the periphery of a pair of gearsforming the embodiment of the invention shown in FIG. 2, partiallydeveloped in a plane;

FIG. 3b is a sectional view along line 3(b)--3(b) of FIG. 3adiagrammatically illustrating the tooth contact pattern between thegears of the first embodiment at a chosen position along the face widthof the gears;

FIG. 3c is a sectional view along line 3(c)--3(c) of FIG. 3adiagrammatically illustrating the tooth contact pattern between thegears of the first embodiment at a chosen position along the face widthof the gears;

FIGS. 4a and b show alternative tooth profiles for the gears of thefirst embodiment of the invention; and

FIG. 5 is a perspective view of a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, FIG. 1a diagrammatically shows the peripheryof a pair of normal involute gears (10, 11) partially developed in aplane to illustrate meshing tooth contact between the teeth of the gears(10, 11). In a known gearing mechanism such as this, the teeth of adriving gear 10 (shown by shading hatched lines) mesh with the teeth ofa driven gear 11 such that the leading flanks 10' of the teeth of thedriving gear 10 contact with the trailing flanks 11" of respective teethof the driven gear 11 in order to transmit drive from the driving gear10 to the driven gear 11. A gap, referred to as back-lash, existsbetween the trailing flank 10" of each tooth of the driving gear 10 andthe leading flank 11' of a rearward or trailing tooth of the driven gear11.

FIG. 1b shows a profile of the teeth of the gears comprising the gearingmechanism of FIG. 1a. The teeth are of conventional profile which willbe contrasted with the desired profiles of teeth for the toothedelements of the embodiments of the present invention as hereinafterdescribed.

Backlash can be a problem on transmissions that are unidirectional,particularly where light loads are encountered. Under such conditionsthe teeth of a driving gear can intermittently lose contact with theteeth of a driven gear due to slight fluctuations in load, and gearrattle can result. This is, in fact, commonly encountered in automotiveapplications. Backlash is, however, a particular problem in abi-directional transmission system in which, on reversal of the drive,the driven gear 11, which now becomes the driving gear, must rotate,albeit by a small amount equivalent to the backlash, before drive in thereverse direction is established. This movement of the driven gear 11(now the driving gear) prior to establishment of transmission drive isalso referred to as back-lash and is undesirable because it can createtransmission shocks which can result in damage to the gear teeth.

The problem of back-lash is also not particularly a problem in powertransmission systems but creates grave problems in providingtransmission systems in the form of gearing mechanisms for precisecontrol of the location of a structure, for example, in robotics,machine tool apparatii, or measuring devices.

FIG. 2 shows a perspective view of a gearing mechanism 20 in accordancewith a first embodiment of the present invention. The gearing mechanismcomprises first and second toothed elements in the form of gear wheels(22, 24), each being shown with a single tooth (26, 28) for sake ofclarity. The teeth (26, 28) on the gear wheels (22, 24) are of identicalform, being regularly undulated and each generally in the form of an"S"-shape in a transverse direction. Hence, as seen in FIG. 3, the curveformed by each tooth (26,28) has a maxima or peak P and a minima orvalley V each located between the endpoints of the curve. The pair ofendpoints formed by each tooth also define end points of an imaginarystraight line. The maxima defines a maximum transverse distance ofextension of said curve from a first side of said imaginary line anddefines a point of contact with one of the two opposing adjacent teethof the other element, and said minima defines a maximum transversedistance of extension of said curve from a second side of said imaginaryline and defines a point of contact with the other of the two opposingadjacent teeth of the other element. The teeth of the first gear wheel22 are of different hand when compared to those on the second gear wheel24 such that, when meshing, each tooth (26, 28) contacts at a point ofcurvature on a drive side and contacts at a reversely directed point ofcurvature on a driven side.

The pitch of the teeth (26, 28) on the gear wheels (22, 24), that is asmeasured from the leading point of curvature to the trailing point ofcurvature, preferably exceeds, by a small predetermined amount, thethickness of the tooth space into which the teeth mesh such that thegear wheels (22, 24) are sprung into meshing engagement and the"S"-configuration of the teeth (26, 28) deform slightly on both gears,thereby preventing back-lash. The teeth are of slender proportions tofacilitate the springing of the gears into meshing.

Whilst it is preferred that all of the teeth (26, 28) on each of thegears (22, 24) have an undulated form as aforesaid, it will beunderstood that the gearing mechanism will still prevent back-lash evenwhere only some of the teeth on one of the gear wheels has an undulatedform, provided that at any point in time, at least one tooth ofundulated form is in meshing contact with radially and axially adjacentteeth of the other gear wheel. Indeed, both gear wheels may comprise acombination of teeth of conventional form and teeth having an undulatedform as aforesaid.

FIG. 3a more readily illustrates the meshing contact between the teeth(26, 28) of the first and second gear wheels (22, 24) of the gearingmechanism of FIG. 2. It can be seen that the teeth 26 of the first gear22 wheel (shown by shaded lines) mesh with those of the second gearwheel such that each tooth makes at least one point of contact with eachof two adjacent teeth on the other gear wheel.

FIGS. 3b and 3c respectively show the pattern of contact between teethof the respective gear wheels at selected positions along the face widthof the teeth.

Whilst it is envisaged that the teeth and gear wheels will bemanufactured by conventional means such as machining from metal, theteeth, and the gear wheels for that matter, may be formed from anysuitable material including rubber and plastics materials. However, itis envisaged that the materials from which the teeth are formed willhave greater rigidity than the materials utilised in the prior artgearing mechanisms as aforementioned thereby avoiding the problem ofslippage between the meshing gear wheels.

FIGS. 4a and 4b illustrate two preferred tooth profiles for the presentinvention. The tooth profile of FIG. 4b is, however, only suitable wherethe mating gear has teeth having a profile as illustrated in FIG. 4a.FIG. 4a illustrates a gear face depth which is higher than normal toachieve the correct proportions to function as aforementioned and toprovide the gear teeth with some resilience.

FIG. 5 illustrates a second embodiment of the invention which is in theform of a rack 40 and pinion 42 gear. However, back-lash is eliminatedin this embodiment in an identical manner to that described above.

In the embodiments of the invention, the gears may be spur or helical,but most likely have a modified involute tooth shape. The tooth willpreferably have a high aspect ratio and be of fine pitch. It is probablethat the top or radially outermost 50 to 60% of the tooth only willcontact during meshing, but the remainder of the tooth depth willcontribute to the flexibility required to enable contact on both flanks,without exceeding root or tooth flank stresses.

The gearing mechanisms of the present invention provide a means forachieving accurate location of any structure that is positioned bygears, with no loss of movement at the point of reversal. This isparticularly useful in applications such as robotics, machine tools andmeasuring devices.

In addition, the gearing mechanisms of the present invention reducenoise and high dynamic tooth loads which can be present due tofluctuating torque conditions accentuated by back-lash or "wind-up"(accumulated back-lash).

I claim:
 1. A gearing mechanism comprising: meshing toothed elements,wherein at least some of the teeth of one of said elements have anundulated longitudinal transverse section whereby said elements can bemeshed such that at any one time one of the teeth having an undulatedlongitudinal transverse section makes at least one point of contact witheach of two opposing adjacent teeth of the other element, and wherein acurve formed by each of the teeth having an undulated transverse sectionis generally S-shaped and has 1) a pair of end points which also defineend points of an imaginary straight line, 2) a maxima positioned betweensaid end points, and 3) a minima positioned between said end points,wherein said maxima defines a maximum transverse distance of extensionof said curve from a first side of said imaginary line and defines apoint of contact with one of the two opposing adjacent teeth of theother element, and wherein said minima defines a maximum transversedistance of extension of said curve from a second side of said imaginaryline and defines a point of contact with the other of the two opposingadjacent teeth of the other element.
 2. A gearing mechanism as claimedin claim 1, wherein each tooth of said one element has an undulatedlongitudinal transverse section.
 3. A gearing mechanism as claimed inclaim 1, wherein the teeth of the meshing toothed elements areresiliently deformable.
 4. A gearing mechanism as claimed in claim 1,wherein the meshing toothed elements are each generally in the form of agear wheel.
 5. A gearing mechanism as claimed in claim 1, wherein one ofsaid meshing toothed elements is in the form of a gear wheel and theother is in the form of a rack, thus providing a gearing mechanism inthe form of a rack and pinion.
 6. A gearing mechanism as claimed inclaim 1, wherein the pitch of teeth on one meshing toothed elementexceeds by a small amount the spaces between teeth of the other meshingelement.
 7. A gearing mechanism as claimed in claim 1, wherein thegearing mechanism comprises a spur gear.
 8. A gearing mechanism asclaimed in claim 1, wherein only an radially outer portion consisting ofbetween 50% and 60% of the radial height of each of the teeth having anundulated transverse section makes contact with other teeth duringmeshing.
 9. A gearing mechanism as claimed in claim 1, wherein only aradially outer portion consisting of between 50% and 60% of the radialheight of said at least one tooth makes contact with other teeth duringmeshing.
 10. A gearing mechanism comprising:meshing toothed elements,wherein at least each tooth of one of said elements have an undulatedlongitudinal transverse section, whereby said elements can be meshedsuch that at any one time one of the teeth having an undulatedlongitudinal transverse section makes at least one point of contact witheach of two opposing adjacent teeth of the other element, and whereineach tooth of the other element has a corresponding conversely undulatedlongitudinal transverse section.
 11. A gearing mechanism comprising:(A)a first toothed element presenting a plurality of teeth; and (B) asecond toothed element presenting a plurality of teeth meshing with theteeth of said first toothed element, wherein at least one of the teethof said first toothed element has an undulated longitudinal transversecross section and contacts simultaneously at least two opposing teeth ofsaid second toothed element, and wherein said undulated longitudinalcross section of said at least one tooth on said first toothed elementis generally S-shaped such that a curve formed by said at least onetooth has 1) a pair of end points which also define end points of animaginary straight line, 2) a maxima positioned between said end points,and 3) a minima positioned between said end points, wherein said maximadefines a maximum transverse distance of extension of said curve from afirst side of said imaginary line and defines a point of contact withone of the two opposing adjacent teeth of the second toothed element,and wherein said minima defines a maximum transverse distance ofextension of said curve from a second side of said imaginary line anddefines a point of contact with the other of the two opposing adjacentteeth of the second toothed element.
 12. A gearing mechanism,comprising:(A) a first toothed element presenting a plurality of teeth;and (B) a second toothed element presenting a plurality of teeth meshingwith the teeth of said first toothed element, wherein at least one ofthe teeth of said first toothed element has an undulated longitudinaltransverse cross section and contacts simultaneously at least twoopposing teeth of said second toothed element, wherein each of theopposing teeth of said second toothed element has an undulatedlongitudinal transverse cross section, wherein the undulations of theopposing teeth of said second toothed element extend conversely to theundulations of the at least one tooth of said first toothed element. 13.A gearing mechanism as defined in claim 11, wherein all of the teeth ofeach of said first and second toothed elements have an undulatedlongitudinal transverse cross section.
 14. A gearing mechanism asdefined in claim 13, wherein the pitch of the teeth of said firsttoothed element, as measured from a leading point of curvature to atrailing point of curvature of the undulations of the teeth, exceeds thethickness of spaces formed between adjacent teeth of said second toothedelement.
 15. A gearing mechanism as defined in claim 11, wherein saidfirst and second toothed elements both comprise gear wheels.
 16. Agearing mechanism as defined in claim 11, wherein said first toothedelement comprises a gear wheel and said second toothed element comprisesa rack.
 17. A gearing mechanism comprising:(A) a first toothed elementpresenting a plurality of teeth; and (B) a second toothed elementpresenting a plurality of teeth, whereinspaces are formed between theteeth of said second toothed element and receive the teeth of said firsttoothed element, wherein each of the teeth of said first toothed elementhas an undulated longitudinal transverse cross section, wherein each ofthe teeth of said second toothed element has an undulated longitudinaltransverse cross section, wherein the undulations of the teeth of saidsecond toothed element extend conversely to the undulations of the teethof said first toothed element, wherein the teeth of said first toothedelement mesh with the teeth of said second toothed element such thateach of the teeth of said first toothed element has a first point ofcontact with a first tooth of said second toothed element and a secondpoint of contact with a second tooth of said second toothed element,said first and second points of contact being spaced tranversely fromone another, and wherein the pitch of the teeth of said first toothedelement, as measured from a leading point of curvature to a trailingpoint of curvature of the undulations of the teeth, exceeds thethickness of the spaces formed between the adjacent teeth of said secondtoothed element, such that said first and second toothed elements aresprung into meshing engagement with one another and such that theundulations of the teeth of said first and second elements deform,thereby preventing backlash.
 18. A gearing mechanism as defined in claim17, wherein each of said teeth is generally S-shaped such that a curveformed by each of said teeth has 1) a pair of end points which alsodefine end points of an imaginary straight line, 2) a maxima positionedbetween said end points, and 3) a minima positioned between said endpoints, wherein said maxima defines a maximum transverse distance ofextension of said curve from a first side of said imaginary line anddefines a point of contact with one of the two opposing adjacent teethof the other element, and wherein said minima defines a maximumtransverse distance of extension of said curve from a second side ofsaid imaginary line and defines a point of contact with the other of thetwo opposing adjacent teeth of the other element.